On Translucent Yttria Stabilized Zirconia Ceramics:

On Translucent Yttria Stabilized Zirconia Ceramics:

Mechanical Considerations, Phase Transformation, and Cement Choices

Sebastian Franco-Tabares

Department of Prosthodontics/Dental Materials Science Institute of Odontology

Sahlgrenska Academy, University of Gothenburg Sweden

Gothenburg 2020

Cover illustration:

“Crown of The Sun” by Sebastian Franco-Tabares.

The central circle of the cover (“The Sun”) is used under the license creative commons 4.0 (CC 4.0). The image corresponds to the first visualization of The Sun’s surface generated by the Daniel K. Inouye solar telescope in Hawaii.

Illustrations in the thesis made by Sebastian Franco-Tabares

On Translucent Yttria Stabilized Zirconia Ceramics:

Mechanical Considerations, Phase Transformation, and Cement Choices

© Sebastian Franco-Tabares 2020 sebastian.franco.tabares@gu.se ISBN 978-91-7833-862-7 (PRINT) ISBN 978-91-7833-863-4 (PDF) Printed in Borås, Sweden 2020 Printed by Stema Specialtryck AB

SVANENMÄRKET

Trycksak 3041 0234

On Translucent Yttria Stabilized Zirconia Ceramics:

Mechanical Considerations, Phase Transformation, and Cement Choices

Sebastian Franco-Tabares

Department of Prosthodontics/Dental Materials Science, Institute of Odontology. Sahlgrenska Academy, University of Gothenburg

Gothenburg, Sweden

ABSTRACT

New zirconia formulations (4Y and 5Y-zirconias) have been introduced for prosthodontic therapy and are commercialized as “translucent” zirconias. No clinical information providing their survival rates or complications has been provided to the date. This has investigated thesis aimed to investigate different variables regarding some of their mechanical aspects, crystallographic and morphological characterization and alternatives for their cementation.

A finite element analysis was performed in order to observe the stress distribution on a monolithic translucent zirconia crown when cemented with different cements. The bonding potential of a 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP)-based commercial cement to two translucent zirconias was assessed using Raman spectroscopy (RS), Fourier- transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). A shear bond strength test was complementary to these analytical methods. The crystallographic structure and surface morphology of two translucent zirconias was estimated by X-ray diffraction (XRD) and interferometry after airborne-particle abrasion (APA) and polishing.

The effect of the cement and air-borne particle abrasion and on the fracture strength of monolithic 4Y-zirconia crowns were also evaluated. And lastly, fracture toughness and hardness were estimated by Vickers hardness measurements.

The stress distribution on a monolithic translucent zirconia crown seems to be unaffected by the cement material. Bonding to 4Y and 5Y-zirconias seems plausible by using a 10-MDP- based cement, nonetheless, thermocycling seems to reduce the bond's strength. The APA protocol produced monoclinic zirconia and was inversely related to the yttria content.

Polishing did not cause any monoclinic zirconia. The surface area and surface roughness were increased after the APA protocol. The fracture strength of the monolithic 4Y-zirconias was unaffected by the APA protocol and the cement choice. Differences in fracture toughness and hardness were observed.

Within the limitations of the studies presented in this thesis, it could be summarized that the translucent zirconias differentiate themselves from the traditional formulations, specially 5Y- zirconias in fracture toughness. Special caution is recommended in their clinical use.

Nevertheless, bonding seems possible via a 10-MDP based cement.

Keywords: zirconia, translucent, yttria, air-borne particle abrasion, 10-MDP, cement

ISBN 978-91-7833-862-7 (PRINT)

ISBN 978-91-7833-863-4 (PDF)

with a special dedication to the daily bravery of my beloved Siri, the courageous wisdom of my mother, Luz Angela, the unconditional presence of my father, Fabio, and the outstanding discipline of my brother, Santiago.

“My dear, accept this dedication [. . .] Who you are, I know not;

where you are, I know not; what your name is, I know not. Yet you are my hope, my joy, my pride, and my unknown honor.”

Søren Kierkegaard

The Crowd is Untruth, Edifying Discourses in Diverse Spirits,

Copenhagen, Denmark 1847.

SAMMANFATTNING - SVENSKA

Nya zirkoniamaterial (4Y- och 5Y-zirkonias) har introducerats för protetisk behandling och är marknadsförda som ”translucenta zirkoniamaterial”; emellertid, saknas information gällande materials överlevnad och kliniska komplikationer.

Denna avhandling har studerat olika variabler gällande mekaniska aspekter, kristallografiska och morfologiska karakteriseringar och effekten av olika cementval för ett par zirkoniamaterial.

En datorsimulering (finitelementanalys) gjordes för att visualisera spänningsfördelningen hos monolitiska translucenta zirkoniakronor cementerade med olika cementtyper. Bindningspotentialen med ett 10-metakryloyloxydecyl- dihydrogen-fosfat (10-MDP)-baserad cement till 4Y- och 5Y-zirkoniamaterial undersöktes och studerades med hjälp av Raman och Fourier-transform infraröd spektroskopi och röntgenfotoelektronspektroskopi (XPS). Ett skjuvtest (SBS) kompletterade de nämnda analytiska metoderna. Den kristallografiska strukturen och ytmorfologin utvärderades med hjälp av röntgendiffraktion och interferometri efter blästring med aluminapartiklar (Al 2 O 3 ) och polering. Effekten av cementtyp och blästring på frakturstyrkan av 4Y monolitiska singelkronor utvärderades också.

Fraktursegheten och hårdheten beräknades med Vickers hårdhetmätning.

Resultaten visade att spänningsfördelningen i en monolitisk translucent zirkoniakrona är opåverkad av cementmaterialet. Bindning till 4Y- och 5Y- zirkoniamaterial förefaller möjligt med ett 10-MDP-baserad cement, men bindningsstyrkan minskar efter termocykling. Blästring skapade ökad mängd monoklinisk zirkonia och var omvänt relaterad till yttria-innehållet. Polering gav inte upphov till ökad mängd monoklinisk zirkonia. Blästring resulterade i en ökad ytråhet och ytförstoringsgrad, men frakturstyrkan av monolitiska 4Y- zirkoniakronor påverkades inte av cementval eller blästring. Fraktursegheten var omvänt proportionell till yttria-innehållet.

Med hänsyn taget till dessa in vitro-studiers begränsningar kan det ändå konstateras att, de nya translucenta zirkoniamaterialen skiljer sig från de traditionella, speciellt 5Y-zirkoniamaterial i frakturseghet. Försiktighet rekommenderas i deras klinisk användning. Emellertid, bindning verkar möjligt med ett 10-MDP baserad cement till de translucenta zirkoniamaterialen.

Nya zirkoniamaterial (4Y- och 5Y-zirkonias) har introducerats för protetisk behandling och är marknadsförda som ”translucenta zirkoniamaterial”; emeller- tid, saknas information gällande materials överlevnad och kliniska komplikationer.

Denna avhandling har studerat olika variabler gällande mekaniska aspekter, kristal- ORJUDÀVNDRFKPRUIRORJLVNDNDUDNWHULVHULQJDURFKHIIHNWHQDYROLNDFHPHQWYDOI|U

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efter termocykling. Blästring skapade ökad mängd monoklinisk zirkonia och var omvänt relaterad till yttria-innehållet. Polering gav inte upphov till ökad mängd monoklinisk zirkonia. Blästring resulterade i en ökad ytråhet och ytförstoringsgrad, men frakturstyrkan av monolitiska 4Y-zirkoniakronor påverkades inte av cementval HOOHUEOlVWULQJ)UDNWXUVHJKHWHQYDURPYlQWSURSRUWLRQHOOWLOO\WWULDLQQHKnOOHW

Med hänsyn taget till dessa in vitro-studiers begränsningar kan det ändå konstateras DWWGHQ\DWUDQVOXFHQWD]LUNRQLDPDWHULDOHQVNLOMHUVLJIUnQGHWUDGLWLRQHOODVSHFLHOOW

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till de translucenta zirkoniamaterialen.

LIST OF STUDIES

The following studies are the structural base of this thesis and are denoted in the text by their Roman numerals.

I.
Franco-Tabares S, Stenport VF, Hjalmarsson L, Johansson CB. Limited Effect of Cement Material on Stress Distribution of a Monolithic Translucent Zirconia Crown: A Three-Dimensional Finite Element Analysis. Int J Prosthodont. 2018 Feb;31(1):67–70.

II.
Franco-Tabares S, Stenport VF, Hjalmarsson L, Tam PL, Johansson CB. Chemical Bonding to Novel Translucent Zirconias: A Mechanical and Molecular Investigation. J Adhes Dent. 2019;21(2):107–16.

III.
Franco-Tabares S, Wardecki D, Nakamura K, Ardalani S, Hjalmarsson L, Stenport VF, Johansson CB. Effect of airborne-particle-abrasion and polishing on novel translucent zirconias: Surface morphology, phase transformation and insights into bonding. In manuscript (Accepted, Journal of Prosthodontic Research, 2020)

IV.
Franco-Tabares S, Hjalmarsson L, Kvam K, Johansson CB, Stenport VF. Effect of the cement and airborne particle abrasion on the fracture strength of translucent monolithic zirconia crowns:

Remarks on their fracture toughness and hardness.

In manuscript (Submitted to the Journal of

Prosthetic Dentistry, 2019)

A BBREVIATIONS ... IV

1 I NTRODUCTION ... 1

1.1 Dental ceramics ... 3

1.2 Zirconia dental ceramics ... 5

1.2.1 Translucent zirconias ... 7

1.2.2 Mechanical and optical properties ... 8

1.2.3 Chemical and biological properties ... 10

1.3 Dental cements ...13

1.4 Zirconia single crowns...16

1.4.1 Effect of the cement... 16

1.4.2 Bonding to zirconia ... 18

2 A IM ...21

3 M ATERIALS AND METHODS ...22

3.1.1 Mechanical analyses... 24

3.1.2 Chemical Analyses ... 30

3.1.3 Surface morphology analysis ... 32

3.1.4 Statistical methods ... 33

4 R ESULTS ...34

4.1.1 Mechanical Analyses ... 34

4.1.2 Chemical analyses ... 38

4.1.3 Surface morphology analysis ... 43

5 D ISCUSSION ...45

5.1.1 Effect of the cement... 45

5.1.2 Fracture toughness and chemically assisted crack growth ... 47

5.1.3 Effect of air-borne particle abrasion (KAPA) ... 54

5.1.4 Artificial ageing: Thermocycling and storage in water. ... 58

5.1.5 The Zr-O-P bond... 59

5.1.6 The nomenclature of zirconias: The use of the adjectives “translucent”, “high”, “super”, and ultra” ... 62

6 C ^ONCLUSIONS ...64

7 F UTURE PERSPECTIVES ...65

A CKNOWLEDGEMENTS ...66

R EFERENCES ...69

A PPENDIX ...90

Zirconia Zirconium dioxide (ZrO 2 )

3Y 3 molecular percentage (3 mol%) yttria 4Y 4 molecular percentage (4 mol%) yttria 5Y 5 molecular percentage (5 mol%) yttria Yttria Yttrium oxide (Y 2 O 3 )

LTD Low-temperature degradation TZP Tetragonal zirconia polycrystal PSZ Partially stabilized zirconia FSZ Fully stabilized zirconia APA Air-borne particle abrasion

KAPA Air-borne particle abrasion according to Kern et al:

0.1–0.25 MPa pressure for 15 seconds using 50 μm alumina particles at a distance of 10–12 mm. With a last cleaning step using 99% isopropyl alcohol in ultrasound.

°C Degrees Celsius

MPa Megapascals GPa Gigapascals N Newtons Å Ångströms nm Nanometres

m Micrometres mm Millimetres

CAD Computer assisted design

CAM Computer assisted manufacture

Alumina Aluminum oxide (Al 2 O 3 ) SBS Shear bond strength

FTIR Fourier transform infrared spectroscopy

RS Raman spectroscopy

XPS X-ray photoelectron spectroscopy XRD X-ray diffraction

NMR Nuclear magnetic resonance

SEM Scanning electron microscopy EDX Energy-dispersive X-ray spectroscopy eV Electronvolts

KeV Kiloelectronvolts FEA Finite element analysis ANOVA Analysis of variance

.stl Stereolithography format file ISO International standard organization

Vm Volume fraction of the monoclinic phase in relation to the tetragonal phase.

wt% Weight percentage

Sebastian Franco-Tabares

1
INTRODUCTION

Prosthodontics is a discipline of dentistry that is especially concerned with the physical, chemical, and biological properties of dental materials. Typically, this concern is cemented on the premise that

“treatment planning, rehabilitation and maintenance” are partially material-dependent, although, the goal of prosthodontic treatment is more or less the same regardless of the materials used: “[to restore]

the oral function, comfort, appearance and health of patients with missing teeth and or maxillofacial tissues” (1). In general terms, patients receiving prosthodontic treatment report an improvement in mastication, smile aesthetics, and satisfaction with their mouth (2).

This thesis examines a specific prosthodontic therapy – tooth supported single crowns. This therapy involves “the replacement of missing tooth structure by surrounding the remaining structure by a material” (1). The treatment is multistep, involving the technical expertise of the dentist and the specific knowledge of the dental technician who fabricates the crown for the patient. The treatment ends with the cementation of the crown. A variety of materials – e.g., ceramics, metals, and polymers – are typically used to replace lost tooth structure (1). Similarly, a variety of cements are available to secure the crown onto the remaining dental structure.

In Sweden, single crowns are registered as code 800 (Permanent tooth-supported crown, one per jaw) and code 801 (Permanent tooth-supported crown, many in the same jaw). The latter code incorporates crowns that are units of bridges and therefore are out of the scope of this thesis. Data provided by The Swedish Social Insurance Office (Försäkringskassan) shows that during the last four years approximately 190,000 single crowns were cemented every year (Figure 1), with a slight decrease every year. Of these crowns, 30%

are registered within the public healthcare system, and the remaining

70% are registered within in private practices (3).

Figure 1. Number of single crowns (code 800) in the last four years in Sweden.

Most of the patients who undergo single crown treatment are between 50 to 79 years old (Figure 2) and are relatively unusual among young adults (20–29 years). As can be observed, single crown treatments are increasingly more common among senior and elderly adults 70 years.

Figure 2. Distribution by age (code 800).

Since 2015, the median price for a single crown has been between

6000 and 6500 Swedish crowns (SEK) (approximately USD 650) in

both the public and private sectors. When compared to other dental

procedures, single crowns are estimated to be the third most with

respect to SEK per year collected: approximately SEK 1,200,000,000

(USD 130 million) are collected per year for single crown treatments

(3).

Sebastian Franco-Tabares

1.1
DENTAL CERAMICS

Ceramics are compounds that consist of a metal/metalloid element and a non-metal element such as oxygen (O). Oxygen, the fuel for most life on this planet, enables, among others, silicon (Si), aluminum (Al), or zirconium (Zr) to become their ‘antithesis’. That is, these oxides (silica, alumina, and zirconia) are on the opposite side of the spectrum regarding their physical, chemical, and mechanical properties (4).

Ceramics were adopted in dentistry at the end of the 18 ^th century. The introduction itself into dentistry came with the improvement of the Chinese porcelain recipe (by adding feldspar and decreasing the lime content). This recipe allowed for the production of more translucent porcelains and indirectly dental prostheses. Two Parisian dentists – Nicolas Dubois de Chémant and Guissepeangelo Fonzi – may be responsible for this important step (5).

Dental ceramics are categorized by composition: as particle-filled glasses and polycrystalline ceramics (4). Particle-filled glasses (i.e., glass-ceramics) are amorphous (i.e., non-crystalline). Their main component is feldspathic glass, where the Si-O-Si bond is the most abundant of the amorphous 3D structures (4) (Figure 3). Feldspathic glass includes two important fillers: leucite and lithium disilicate. In a 1962 patent application, leucite was described as the key component for matching the thermal expansion of virtually any dental alloy, making it possible to veneer metal substructures (the invention of metal-ceramic restorations) (6). Lithium disilicate, a more recently introduced filler, improves mechanical properties by reacting when pressed into the feldspathic glass matrix. This mechanism is not new, Dicor (Dentsply, USA), the first all-ceramic material, also used this type of reaction although a different filler (4). In contrast, polycrystalline ceramics have a defined atomic structure.

Theoretically, the higher grade of organization represents an increase in mechanical properties but a decrease in translucency by providing light a denser and more arranged medium to pass through (4).

Alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) are the most representative of

this group. Alumina and zirconia were used in medical science during

the 1970s and late 1980s, respectively, as alternative materials in hip-

replacement surgery (7, 8). However, due to their high melting point

(>2000 °C), their customization for individual dental restorations was

industrially unfeasible. It was not until the CAD/CAM technology

was adopted in dentistry that customized restorations could be made;

prior to this, mass production of standardized sizes was used for polycrystalline ceramics.

Figure 3. Illustration of the atomic structure of

a) particle filled-glass ceramic (feldspathic ceramic) (based on ( 5))

b) polycrystalline (zirconia).

Sebastian Franco-Tabares

1.2
ZIRCONIA DENTAL CERAMICS

An article published in the journal Nature, “Ceramic Steel?”, marked the beginning for zirconia ceramics. In this article, Garvie et al.

describe “transformation toughening” by making an analogy with a type of steel that undergoes an internal phase transformation that increases its resistance to fracture. In the article, the authors provided experimental data supporting zirconia’s phase transformation as a basis for its unusual strength (9).

Zirconia’s atoms are organized into three units (i.e., allotropes). They are found at different temperatures in pure zirconia: monoclinic (m) at room temperature, tetragonal (t) at 1170 °C, and cubic (c) above 2370 °C (Figure 4). For each phase, the unit cell volumes are 140 Å ³ for monoclinic (10), 68 Å ³ for tetragonal, (11) and 138 Å ³ for cubic (12). An additional phase has been reported, the rhombohedral.

Initially reported by Scott in 1977 (13), later described in ceramic engineering (14–17) and recently reported in dental research literature (18–20). The rhombohedral phase may resemble a symmetrically distorted cube (21). However, unit cell volume estimations are inconsistent. Some authors report it to be 2.5% larger than the cubic unit cell (21) and others 1–2% larger than the tetragonal unit cell (17).

However, Scott reported a cell unit volume of 747 Å ³ (13). The rhombohedral phase is theorized to be a distorted cubic phase (15,18); however, some studies report the rhombohedral phase to be a distorted tetragonal phase (17,19,20). Moreover, some authors regard the rhombohedral phase as both a distorted tetragonal phase and a cubic phase (14,18). In addition, this rhombohedral phase may be a reorganization of the atoms in the crystal units with the possibility to reverse to tetragonal or cubic after annealing (22).

Different metal oxides have been used to stabilize tetragonal and

cubic zirconia at room temperature. Oxides of calcium, magnesium,

and cerium are some examples (9,23). However, yttria (Y 2 O 3 )

provides a more effective combination of toughness and strength in

relation to the molecular percentage (mol%) used (23). Used at 3

mol%, zirconia constituted mainly by tetragonal units are produced

(traditional formulation or 3Y-zirconia) (24).

Figure 4. Zirconia’s allotropes

The use of zirconia in dentistry occurred in the early 1990s when it was first used as a filler for porcelain (25). Later in the same decade, zirconia began to be used in ceramic brackets (26) and post systems (27).

As previously mentioned, the progressive adoption and improvement

of the CAD/CAM technology facilitated the production of

individually customized restorations such as crowns. The first article

describing a method to produce monolithic polycrystalline crowns

was published in 1999 (28) and the first clinical study, which

presented two-year results of the clinical performance of zirconia-

based crowns, was published in 2009 (29).

Sebastian Franco-Tabares

1.2.1
Translucent zirconias

The opacity of zirconia compared to glass-ceramics or natural teeth is a shortcoming when used as monolithic material (30). It has been reported that the phenomenon behind this may be two-fold: a) the presence of pores, defects, and light-scattering dopants and b) zirconia-specific light-scattering (30).

Yamashita and Tsukuma reported that pore size and porosity significantly affect the translucency of yttria stabilized zirconia (31).

A porosity of 0.1% with a pore diameter of 100 nm decreases the translucency to 0% at a thickness of 1 mm (31). However, other authors have documented that at a porosity of 0.01% translucency can still be achieved at 1-mm thickness even if the pore size is greater than 100 nm (32). Alumina has been used as a dopant for zirconia ceramics given its capacity to increase the final density (33). When used at >0.1 mass%, densities approaching the theoretical maximum are reached at a sintering temperature of 1300 °C (33). Nevertheless, the refraction index of alumina differs compared to the refraction index of zirconia (34). Zhang et al. showed that alumina-doped 3Y- zirconia presents three times more light scattering than alumina-free 3Y-zirconia (34).

The presence of various phases in zirconia ceramics gives rise to the second obstacle regarding the opacity of zirconia-based restorations.

The tetragonal and the cubic phases have different refraction indexes, making the ceramic birefringent or optically anisotropic (32,35,36).

The refraction index is a comparison of the speed of light travelling though air with the speed of light travelling through a material. The cubic phase has a refraction index of 2.16 and the tetragonal phase has a refraction index of 2.20 (36). Additionally, the refraction index for zirconia is higher compared to that for other ceramic systems.

Refractive indexes reported in the literature are 2.20 for zirconia (30), 1.76 for alumina (30), 1.51 for leucite (37), and 1.50 for feldspathic glass (dental porcelain) (38). Summarizing, light travels 2.16-2.20 times slower when passing through zirconia than when passing through air.

Different approaches have been proposed to solve the opacity

problem of zirconia. For example, Zhang modeled a theoretical

minimum grain size to produce translucent 3Y-zirconia (30). Using a

Rayleigh scattering model, the author recommends a maximum grain

size of 82 nm to produce the same translucency as dental porcelains at a thickness of 1.3 mm (30). Other authors propose inclusion of large cation radius dopants such as La 2 O 3 and Nd 2 O 3 in traditional formulations (39,40). The addition of those oversized dopants is suggested to improve the translucency of traditional formulations up to 42% of their initial translucency (40). These dopants, used at 0.2%

mass, seem to be localized at the outermost region of the grain boundaries. The mechanism may be a reduction in the birefringence of zirconia by grain boundary segregation (40).

Increasing the content of yttria in terms of mol% has been another alternative (30). Increasing the mol% of yttria up to 4 to 5% is the basis for materials that today are commercially available as

“translucent zirconias” (4Y- and 5Y-zirconias) (41). Although originally used as a marketing term for zirconia, today “translucent”

is used to describe all types of zirconias, from traditional formulations to the new 4Y and 5Y formulations (41). However, the new formulations, specifically the 5Y-zirconia, report half of the flexural strength and more than half of the fracture toughness of the traditional formulations (41).

1.2.2
Mechanical and optical properties

Zirconia ceramics report a superior flexural strength and fracture toughness compared to other ceramic systems, especially the traditional formulations compared to glass-ceramics. The flexural strength of traditional formulations is  900 MPa, which could be illustrated as follows: fracture at flexure occurs when a 90-kg rugby player “stands” on a square millimeter. This flexural strength is, for example, 10 times higher than the first all-ceramic material (Dicor, Dentsply, USA) (42) and double as much as the flexural strength of lithium disilicate (43). Fracture toughness, refers to the material’s ability to resist fracture provided a preexisting defect. Reported fracture toughness values for yttria stabilized zirconia seem to be inversely proportional to the yttria content (44). For traditional formulations, values are around 5 MPa.m ^1/2 , which is approximately 1.5 times higher than those of lithium disilicate (45). The elastic- modulus, however, has been observed to be constant (210 GPa).

Traditional formulations, which are predominately tetragonal,

present the same elastic modulus as 100% cubic formulations (46,47).

Sebastian Franco-Tabares

Transformation toughening is the mechanism responsible for these mechanical properties (44,48) (Figure 5). The phase transformation from tetragonal to monoclinic phase involves a shear strain of 8%

and a volume expansion of 4% (44). This phenomenon results in the arrest of fractures and flaws, stopping their propagation within the material (44). For 4Y- and 5Y-zirconias a decrease in their flexural strength and fracture toughness is reported (45,49); however, limited literature addresses this topic.

Figure 5. Transformation toughening.

Zirconia’s birefringence and high refraction index results in typical low translucency values compared to other ceramic systems, as previously mentioned. Traditional formulations report translucencies at 1.0-mm thickness of 20%, 5Y-zirconia of 23%, and lithium disilicate of 27% (50). At a reduced thickness of 0.5 mm, traditional formulations have a translucency of 28% and 5Y-zirconia has a translucency of 32%, both contrasting with lithium disilicate’s translucency of 40% at the same thickness (50).

Grain size is related to both optical and mechanical properties of

zirconia. For traditional formulations, an increase in grain size is

related to a decrease in mechanical properties (51) and translucency

(30). However, 5Y-zirconia formulations show variability in grain size

(350 nm to 810 nm) (49) but still exhibit an increased translucency

(50).

1.2.3
Chemical and biological properties

A particular phenomenon occurs in zirconia ceramics, especially the traditional formulations with high tetragonal content. Given that a stabilizer (i.e., yttria) is used to produce tetragonal units at room temperature, the ceramic itself is called “metastable” as the ceramic can phase transform in a defined setting. When this transformation occurs related to temperature, the phenomenon is termed low- temperature degradation (LTD).

Kobayashi et al. found that zirconia formulations containing 4.5 to 5 mol% yttria could present approximately 20% monoclinic content after 3000 hours at 200–300 °C (52). Nevertheless, formulations containing more or equal than 5% and up to 8.0% mol yttria reported 3 to 0% monoclinic content after ageing at the same temperature and for the same period (52). The temperature interval at which this phenomenon happened most rapidly was believed to be a limiting factor with respect to clinical use. In 2001, a certain amount of zirconia-based hip prosthesis started to present catastrophic failure (fracture of the prosthesis). Reports investigating the failed prostheses showed an increased surface roughening and monoclinic phase content after clinical service for as short as 1–2 years (53,54) (Figure 6). Further investigations showed that a certain batch of zirconia hip heads underwent an alternative sintering protocol (23).

This alternative method used a cooling protocol twice as rapid together with the use of a tunnel furnace instead of a closed furnace and slow cooling. The consequences of this faster sintering protocol may have resulted in increased porosities of the zirconia hip heads.

An increased porosity or a decreased density probably was the determining factor that lead to an accelerated LTD (24).

An additional factor associated to LTD is grain size. An increased grain size has been correlated to an increased LTD (55). It is theorized that larger grains are enriched with the predominantly cubic stabilizer, yttria. Other regions, specifically yttria-deficient regions, are destabilized tetragonal regions and more susceptible to LTD (55).

Nevertheless, LTD is not an exclusive problem of zirconia with lower

density or with increased grain size. It has been shown that a certain

amount of LTD is expected to occur in optimally-sintered traditional

zirconias (24). In response to these issues, the International Standard

Organization presented a regulation (ISO 13356) specifying, among

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others, a minimum density ( 6.0 g/cm ³ ) and grain size ( 400 nm) for yttria stabilized zirconia surgical implants.

The new formulations of zirconia are yet to be investigated in this matter. Theoretically, a factor that could be favorable is the increased amount of cubic zirconia that cannot transform to tetragonal and then to monoclinic. Nevertheless, the mechanical properties may be a shortcoming, and for some formulations the larger grain size may be a shortcoming (51).

Figure 6. Low-temperature degradation.

The effect of air-borne particle abrasion (APA) in relation to low- temperature degradation has been documented. Cotic et al. reported that a traditional formulation sintered at 1500 °C that underwent APA with 50-m and 110-m alumina particles at 0.25 MPa results in a decreased formation of monoclinic content (40% less) compared to as-sintered specimens after artificial ageing in an autoclave at 137

°C for 12 and 48 hours (56). Other studies have noted this phenomenon in traditional formulations and have observed that for traditional formulations APA provides a form of protection against LTD (20,57). However, the mechanism is not fully understood (20), and translucent formulations need further exploration.

Studies of zirconia-based materials have reported favorable results in

terms of biological properties. Variables aiming to measure

osseointegration, such as bone-implant contact and peri-implant

bone volume density, are reported to be at least as much as the ones

observed for titanium in a variety of animal models (white rabbits,

mini-pigs, adult pigs, and beagles) for 2 to 12 weeks (58–63). Clinical

studies including zirconia abutments in peri-implant soft tissue

conditions have not reported any significant differences compared to

titanium abutments in terms of variables such as plaque index,

gingival index, and periodontal probe depth (64–67). Other clinical

studies measuring bacterial adhesion using variables such as bacterial

count and area colonized have not documented any statistical

differences between titanium and zirconia abutments (68–70). It has

been theorized that the biological inherent properties of zirconia may

not interfere in the normal proliferation of fibroblasts, which

facilitates the structuring of a good mucosal barrier (71). Because the

mentioned clinical studies are short; long-term clinical trials are

needed before any valid recommendation can be made. Furthermore,

the new formulations of zirconia have not been investigated

regarding their potential osseointegration, peri-implant soft tissue

reaction, and bacterial colonization.

Sebastian Franco-Tabares

1.3 DENTAL CEMENTS

Dental cements are used in the last step of single crown treatment.

Their main function is to firmly unite the crown with the tooth preparation, which is ultimately the patient. Cements have been present in the daily clinical practice since the use of zinc phosphate at the end of the 19 ^th century (Figure 7) (72). Other cements were gradually introduced into dentistry, specifically with the advancement of polymer and glass technologies. The resin-based cements were introduced at the beginning of the 1960s (73), the glass-ionomers in the mid 1970s (74), and the resin-modified glass-ionomers in the early 1990s (75).

Figure 7. An 1892 report on using zinc phosphate as cement.

Dental cements are categorized into two large groups: water-based and resin-based. The International Standard Organization has separate documents regulating each group (ISO 9917 and ISO 4049, respectively). The water-based cements are set using an acid-base reaction, and the resin-based cements react by polymerization.

Special interest on the properties of various dental cements has

existed since they are the linking agent between the restoration and

the patient. The cements have also been considered as a potential

vehicle for delivering fluoride, for example, adhering chemically to

the dental structure and restoration, stimulating the formation of reparative dentin and impeding the colonization and sometimes re- colonization of the remaining dental structure by bacteria (76).

Some minimum values for compressive and flexural strength have been recommended by the International Standard Organization (Table 1). Compressive strength seems to be of special interest for water-based cements and flexural strength seems to be of special interest for resin-based cements. The standards (9917-1:2007, 9917- 2:2017 and 4049:2019), however, do not explain how these minimum values can be related to clinical success or at least related to clinical studies.

Table 1. Compressive strength and flexural strength of the most representative cements.

Cement Compressive

Strength ISO

requirement Flexural

Strength ISO requirement Zinc

phosphate 69 MPa 50 MPa

(9917-1:2007) 6 MPa Not specified Glass-

ionomer 104 MPa 50 MPa

(9917-1:2007) 15 MPa Not specified Resin-

modified glass- ionomer

115 MPa Not specified 15 MPa 10 MPa (9917-2:2017)

Resin 166 MPa Not specified 90 MPa 50 MPa (4049:2019)

Values obtained from (77)

The biological properties of dental cements are of particular interest given their close proximity to the dentino-pulpal organ and the gingiva. Different cellular models have presented varying results (76).

Zinc phosphate results in a higher human gingival fibroblast viability than glass-ionomer and resin cement (78). However, contrasting results exist regarding rat fibroblasts (79). It is speculated that the determining factor could be the remaining substances produced by the reaction or un-reacted material irrespective of cement (76,78).

Allergic reactions to dental cements have been reported, mostly to

resin-based cements, but these reactions are rare (80,81).

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The bonding capacity of dental cements has also received special attention. Cements containing functional groups that can theoretically bond to the dental structure or to a variety of materials have been developed.

Polyacrylic acid (PAA), present in glass-ionomers, is believed to bond with the hydroxyapatite existing in dentin and enamel (82). The carboxyl functional group (COO ^- ) seems to form an ionic bond with the hydroxyapatite’s calcium (Ca) and is resistant to ultrasound (82).

Other molecules present in resin adhesives – e.g., 4-

methacryloxyethyl trimellitic acid (4-MET), 2-methacryloxyethyl

phenyl hydrogen phosphate (phenyl-P), and 10-methacryloyloxydecyl

dihydrogen phosphate (10-MDP) – can bond to the calcium present

in hydroxyapatite (83). Bonding to glass-ceramics, for example, is

based on the same principle as the bonding of the silica filler particles

to the polymer matrix of common dental composites. A silane is

applied as the coupling agent between the cement and the glass-

ceramic (84).

1.4
ZIRCONIA SINGLE CROWNS

The estimated five-year survival rate of zirconia single crowns is 93.8%, a survival rate statistically equal with metal ceramic single crowns. The two main complications are porcelain chipping (approx.

3%) and loss of retention (approx. 4%) (90). Chipping of the veneering porcelain was a recurrent clinical problem for zirconia- based restorations at the beginning of the last decade (85). Different theories were presented to explain this phenomenon (5): mismatch of the thermal expansion of zirconia and porcelain, for example, or phase transformation from tetragonal to monoclinic at the zirconia- porcelain interface (86). However, these clinical problems appear to be the result of residual stresses accumulated during rapid cooling of the veneering porcelain (85). After a slower cooling protocol was adopted, especially during the last porcelain firing, a decrease in chipping has been reported (5). Given this complication, monolithic zirconia restorations are an alternative to veneered zirconia.

1.4.1
Effect of the cement

The effect of the cement on the fracture strength of monolithic traditional zirconia single crowns seems to be negligible. Nakamura et al. documented that monolithic 3Y-zirconia single crowns had no statistical different fracture strength when cemented with zinc phosphate, glass-ionomer, and a resin cement, even at a reduced thickness of 0.5 mm (87). The effect of the cement in the fracture strength of 4Y and 5Y formulations has not been investigated.

Clinical literature concerning this apparent versatility of veneered 3Y- zirconia can be found in Table 2 (88–97). Two studies reported loss of retention. Örtorp et al. reported loss of retention of 15 crowns with both cements – zinc phosphate (n=2) and resin cement (n=13) (91). Monaco et al. registered two crowns losing retention that were cemented with resin cements (94). The American Academy of Prosthodontics has also registered this apparent versatility (98).

Nonetheless, long-term studies that include both water-based and

resin-based cements are still needed to provide a better understanding

of the topic (98).

Sebastian Franco-Tabares

Table 2. Cements used for traditional zirconia-based single crowns.

Year/Authors/Country/Type of Study, mean

follow-up time Cements used, n=number of crowns 2017/Dogan et al./USA

Prospective, 49 months Dual-cure self-adhesive resin-cement (RelyX Unicem, 3M, USA),

n=20 2015/Seydler and Schmitter/ Germany

Randomized controlled trial, 25 months Self-curing resin-cement

(Multilink Automix, Ivoclar Vivadent, Liechtenstein),

n=30 2014/Gherlone et al./Italy

Retrospective, 36 months Dual-cure self-adhesive resin-cement (RelyX Unicem, 3M, USA),

n=86 2013/Monaco et al./Italy

Retrospective, 60 months Resin cement, n=792 Glass-ionomer, n=235 Zinc-phosphate, n=77 2013/Rinke et al./Germany

Prospective, 36 months Resin-modified glass-ionomer (Dyract Cem plus, DeTrey, USA),

n=52 2012/Sagirkaya et al./Turkey

Randomized controlled trial, 74 months Dual-cure self-adhesive resin-cement (Panavia f2.0, Kuraray, Japan), n=74

2012/Örtorp et al./Sweden

Retrospective, 60 months Zinc phosphate (Phosphacem, Ivoclar Vivadent, Liechtenstein),

n=16

Self-adhesive resin-cement (Rely X Unicem, 3M, USA),

n=200 2012/Vigolo et al./Italy

Prospective, 48 months Glass-Ionomer (Ketac-Cem, 3M, USA), n=39

2010/Beuer et al./Germany

Prospective, 35 months Glass-Ionomer (Ketac-Cem, 3M, USA), n=50

2010/Schmitt et al./Germany

Prospective, 39 months Glass-Ionomer (Ketac-Cem, 3M, USA),

n=17

1.4.2
Bonding to zirconia

There is an interest in creating a stable chemical bond to zirconia that resembles the stable chemical bond for glass-ceramics. The possibility of combining the advantageous mechanical properties of zirconia with a bonding protocol is appealing, especially in the age of minimally invasive dentistry. Moreover, this development could address the main clinical complication – loss of retention. Due to the nature of zirconia, the same protocol of acid etching with hydrofluoric acid (HF) at room temperature and silanization cannot be performed (84). More than 20 methods to create a bond to zirconia (e.g., the use of laser treatments, glass infiltrations, and primers) have been explored (99).

The molecule 10-MDP, designed to have hydrophilic and hydrophobic properties (amphipathic), was initially engineered for dentinal bonding but later investigated as a potential bonding molecule to multiple materials (100). Suzkuki et al., using vibrational spectroscopy, have provided molecular evidence that 10-MDP bonds to silver (Ag) and this bond is stable up to 200 thermocycles (101).

Moreover, the stability of the bond seems to be improved compared to a similar molecule but shorter in nature (2-MDP) (101). Similar findings using vibrational spectroscopy regarding molecular bonding of 10-MDP to chrome (Cr) have also been documented (102).

The possibility of bonding to different substrates means that 10- MDP is categorized as an universal adhesive (103).

More recently, the potential bonding of 10-MDP to zirconia has been investigated. Chen et al. provided molecular documentation of the possible bond Zr-O-P using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Other studies using Fourier-transform infrared spectroscopy (FTIR) reported the adsorption of 10-MDP on traditional zirconia formulations (104–107). The above studies registered peaks at 1083 and 981 cm ^1 , which were assigned to the asymmetrical and symmetrical vibrations of the PO 3 2- functional group from 10-MDP adsorbed on traditional zirconia (104–107).

Another analytical method that has been used to assess the bond of

10-MDP to traditional zirconia has been X-ray photoelectron

spectroscopy (XPS) (106–108). The principal focus of the mentioned

studies has been the analysis of the intermediate oxygen present in

the Zr-O-P bond and specifically its first level of energy (Os1) (106–

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108). Energies corresponding to that possible bond have been shown to exist in the 531.5–531.8 eV interval (109).

Using nuclear magnetic resonance (NMR), some studies have provided insight into how 10-MDP may be adsorbed onto traditional zirconia by the interaction between the P-OH and Zr-OH groups or between P-O ^- and partially positive Zr ⁴⁺ . Moreover, the Zr-O-P bond is resistant to ultrasound when exposed to acetone for 40 min, and being possibly an ionic bond (110). The modification of 10-MDP for improving its affinity to zirconia has also been explored (106). Using acetone or ethanol as solvents may provide a better affinity compared to using water (106). Moreover, if the carbon chain is extended over 10 carbons with the inclusion of a carboxyl (COO ^- ) or a hydroxyl (OH ^- ) group in the middle of the chain, unlike a typical 10-MDP molecule used in traditional zirconia, affinity may be improved (106).

Indirect methods such as mechanical testing have also been used.

Shear bond strengths seem to range between 10–20 MPa (108,111), and tensile bond strengths seem to range between 30–50 MPa (112).

Figure 8. a) the 10-MDP molecule and b) the cations for bonding.

Clinical studies involving 10-MDP have been presented by a research group led by Professor Matthias Kern from the University of Kiel.

The group developed a procedure that involves the air-particle

abrasion of the surface at 0.1–0.25 MPa for 15 seconds using 50-μm

alumina particles at a distance of 10–12 mm. The last cleaning step

uses 99% isopropyl alcohol in an ultrasound cleaner. Subsequently, a

10-MDP-based cement or primer is used. To date, two clinical studies

have investigated clinical outcomes: a ten-year retrospective study

evaluating the survival rate of zirconia-based cantilever resin bonded

fixed dental prostheses (RBFDPs) in the anterior region (113) and a

three-year prospective study reporting the survival rate of zirconia-

based inlay-retained fixed dental prostheses (IRFDPs) in the

posterior region (114). The anterior region had a survival rate of

98.2% after ten years and the posterior region had a survival rate of

100% after three years. However, the anterior region study had a

retrospective design, the posterior region study had a small sample

size (n=23) for the three-year follow-up, and both studies did not

include a control group. Additionally, other research groups need to

confirm the clinical outcomes independently. Moreover, no

information has been reported that describes how to use this protocol

to bond translucent zirconias.

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2
AIM

This thesis investigated some mechanical aspects, phase transformation, and cement considerations for 4Y- and 5Y-zirconias.

The specific aims of this thesis were as follows:

a.
To evaluate the effect of various cements on the stress distribution of a monolithic translucent zirconia crown.

b.
To study the bonding potential of a 10-MDP-based cement for two translucent zirconias.

c.
To assess the effect of an air-particle abrasion protocol and polishing on two translucent zirconias.

d.
To observe the effect of an air-particle abrasion

protocol and cement on the fracture strength of 4Y-

zirconia monolithic crowns.

3
MATERIALS AND METHODS

Zirconias used in this thesis

Three types of zirconias were used in this thesis. They were differentiated from each other by the flexural strength provided by each manufacturer.

Table 3. Trade name, lot number, flexural strength, and manufacturer of the different zirconias used in this thesis.

Trade name Provided Flexural Strength Manufacturer BruxZir 2.0 HT

(lot B1226355) >800 MPa Glidewell, USA DDcubeX ² HS

(lot 6161719002) 1100 MPa DentalDirekt, Germany Copra Smile

(lot IS2189A2) 600 MPa Whitepeaks, Germany

The materials were elementally characterized by energy dispersive X- ray spectroscopy (EDX). One block (3 mm x 3mm x 1mm) of each zirconia was embedded in epoxy resin (EpoFix batch 4138-3380, Struers, Denmark) to facilitate its handling and polishing. The polishing procedure was carried out with SiC papers (Struers, Denmark) (#500, #1000, #1200, #2400, and #4000) until optical finish. Following extensive water rinsing and drying, the three specimens were mounted on aluminum stubs using carbon tape. The elemental information was gathered from five points per specimen and can be found in Table 4.

Varying mean values of yttrium were observed (Table 4). Estimations

of the yttria (Y 2 O 3 ) molecular percentage (mol%) were 2.7 mol% for

BruxZir 2.0 HT, 4.2 mol% for DD cubeX ² HS, and 4.8 mol% for

Copra Smile. However, these values are most likely approximations

given the low electrical conductivity of the specimens and the similar

values for the yttrium and zirconium L
spectral lines (1.924 and

2.044 KeV respectively) (115). This can be observed from the broad

peak initiating from 1.9 to 2.3 KeV (Figure 9). Consequently, the

materials were assumed to be 3, 4, and 5 mol% yttria stabilized

zirconia – BruxZir 2.0 HT, DD cubeX ² HS, and Copra Smile,

respectively.

Sebastian Franco-Tabares

Table 4. Elemental characterization of the zirconias used in this thesis.

Atomic % (SD)

O Zr Y

BruxZir 2.0 HT 67.6% (0.4) 30.7% (0.3) 1.7% (0.1) DD cubeX ² HS 67.4% (0.3) 30.0% (0.2) 2.6% (0.1)

Copra Smile 67.4% (0.6) 29.7% (0.5) 3.0% (0.1)

SD: Standard deviation

Figure 9. EDX spectra of the three zirconias: a) BruxZir 2.0 HT; b)

DD cubeX ² HS; and c) Copra Smile.

3.1.1
Mechanical analyses

Finite element analysis (Study I)

A standard lower molar gypsum die (Fujirock, GC, Japan) was prepared for a single crown using a high-speed handpiece (Ti95, NSK, Japan). The taper of the preparation was measured with a Nikon SMZ800 stereomicroscope (Nikon Instruments, USA), and the image software analysis (NIS-Elements BR 3.2, Nikon Instruments, USA) was performed using a total magnification of 10.

A total of four angles on four surfaces were measured (buccal, lingual, mesial, and distal). The reference point for each angle was 1.0 mm above the finish line as previously described (116). The total occlusal convergence angles were 14° buccolingually and 17° mesiodistally, which are both within the ranges previously recommended to obtain a proper retention and resistance (117,118). Both the preparation and the full anatomic crown were scanned using a laser scanner (3Shape D700, 3Shape A/S, Denmark). Both the marginal gap (MG) and the cement thickness (CT) were modelled based on information provided by clinical findings (119,120). The MG was modelled as 110 μm, and the CT was modelled as 280 μm on the occlusal surface and 110 μm on the axial walls. The thickness of the crown at the central fossa was 1.0-mm and 0.5-mm thick peripherally on the cervical margin. The loading force magnitudes (225 N and 600 N) and their directions (0°

and 19.5°) were obtained from clinical findings (119–122). The number of loading points was described using two conditions: a conventional cusp-to-marginal ridge occlusion resulting in five points and one single loading point in the central fossa. The diameter of the loading points (ø 1.25 mm) was also obtained from the literature (123).

A variety of cements were evaluated: zinc phosphate, glass-ionomer,

resin-modified glass-ionomer, dual-cure resin, calcium aluminate-

based, and a theoretical or conceptual cement. The latter was

intended to represent a cement with mechanical properties

resembling dentin. As the information regarding the mechanical

properties of the periodontal ligament was inconclusive, this

information was omitted. The proportional limit of each material was

selected as the failure parameter because a plastic deformation for the

brittle materials used in the present study simply represents a fracture

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of the material itself (77). For the cements, the proportional limit values were obtained from a reference (77). The calcium aluminate- based cement was provided by the manufacturing company (Doxa AB, Sweden). Information about dentin was obtained from the literature (42). For the monolithic translucent zirconia crown, an average value (1000 MPa) for the flexural strength of various commercially available translucent zirconias was used.

Figure 10. Loading conditions. Every loading condition is a combination of three variables: loading force, angle, and number of contact points.

At the bottom of the abutment, a flat surface parallel to the occlusal

plane of the crown was set as a fixed support. No sliding or separation

was allowed between the interfaces of the components (crown-

cement abutment). The software used to model the CT was Dental

System 2014™ premium (3Shape A/S, Denmark) and Autodesk 3ds

Max 2011 (Autodesk Inc., USA). The average element quality was

0.82 (SD 0.09): 1 was the highest and 0 the lowest quality according

to the simulation software (ANSYS Workbench 2015, SAS IP Inc.,

USA).

In total, the model included 525,734 tetrahedral elements, 829,647 nodes, and 2,488,941 degrees of freedom (three degrees of freedom per node). The total number of simulations was 48 (8 per cement).

Table 5. Elastic modulus and Poisson’s ratio of the materials used.

Material Elastic Modulus, GPa Poisson’s ratio, unitless

Dentin 19 (124) 0.30 (124)

Conceptual cement 18 0.30

Zinc phosphate 13,5 (42) 0.33 (125)

Glass ionomer 7,3 (42) 0.35 (126)

Resin modified glass ionomer 5,1 (42) 0.30 (127) Calcium aluminate-based 4,5 * 0.28 * Dual cure resin 6,5 (128) 0.30 (129)

Zirconia 210 (47) 0.30 (42)

*Information provided by Doxa AB, Sweden.

Shear bond strength (Study II and Study III)

Studies II and III included a shear bond strength (SBS) test based on

the ISO standard 29022:2013 (Figure 11). The standard uses a

notched-edge stylus designed to homogenously distribute the stress

generated at the interface of the adhered materials. The specimens

(n=10) were cylinders (3.0 mm  x 2.0 mm height) of each type of

zirconia produced according to the manufacturers’ instructions. The

specimens were embedded in epoxy resin (EpoFix, batch 4138-3380

for Study I and batch 8348-01 for Study II, Struers, Denmark) and

polished until the zirconia surface was exposed. The polishing

process was carried out using SiC papers (Struers, Denmark) until

optical finish, in Study II #500, #1000, #2000 and #2400. In Study

III until #4000 with final polishing step using a polishing cloth

moistened with a lubricant (DP-Lubricant Blue, batch 5335, Struers,

Denmark) and a polishing suspension (OP-S Suspension, batch 4218-

8333, Struers, Denmark). After polishing, the specimens were cleaned

from any residual polishing particles by ultrasound for 20 minutes

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and rubbed with 99% isopropyl alcohol for three to four seconds.

The specimens were left to dry at room temperature.

In Study II, air particle abrasion of the surface was performed according to Kern’s protocol (KAPA) (0.1–0.25 MPa, 50-μm alumina, 10–12 mm distance, 15 seconds, and cleaning in ultrasound using 99% isopropyl alcohol). In Study III, a first SBS test was performed using the highly polished surface. Composite cylinders of 2.4 mm  x 1.5 mm height were used in Study II and III (Spectrum TPH3 lot 1707000722 for Study II and lot 0986 for Study III) (Dentsply Sirona, USA). In Study III, for the second SBS test, the surface was cleaned with ultrasound for 15 minutes. After the cleaning procedure, they underwent KAPA, and the second SBS test was carried out. Zinc phosphate cylinders (2.4 mm  x 1.5 mm height) were used (liquid, normal setting lot 1101609; powder, normal setting lot 91605022) (Harvard Dental International, Germany). For the manufacture of all the cylinders, a poly methyl methacrylate (PMMA) (White Peaks, Germany) mold was used. For the composite cylinders, a 10-MDP-based cement was used (Panavia F 2.0, lot 000055 both pastes in Study II and Paste A lot 7E0167, Paste B lot 270072 in Study III) (Kuraray, Japan). For the zinc phosphate cylinders, the same zinc phosphate cement was used. The manufacturer’s recommendations were followed. Following the ISO standard 29022:2013, the specimens were stored in deionized water at 37 °C for 24 hours. In Study II, half of the specimens were thermocycled 10,000 times (5

°C–55 °C). Next, the specimens were mounted in a universal testing machine (LRX 9772, Lloyd Instruments, UK). A notched-edge cross head at a rate of 1.0 mm/min was used. The shear bond strength was calculated by dividing the force at de-bonding (N) by the bonding area (4.52 mm ² ).

Figure 11. Illustration of the specimens for the shear bond test.

Fracture strength (Study IV)

The digital models (.stl format) of the crown and abutment from the finite element analysis were used. The monolithic crowns were manufactured from DD cubeX ² HS (DentalDirekt, Germany) zirconia and the abutments from G10, a glass-epoxy laminate that served as a dentin analog. As in the finite element analysis, the crowns were 1.0-mm thick at the central fossa and 0.5-mm thick on the marginal edge. Subsequently, the crowns were divided into two groups: As-sintered (n=15) and KAPA (n=15).

The cementation process was performed in a climate room with a relative humidity of 46% and an approximate temperature of 23 °C.

The process involved three cements: a zinc phosphate (as in Study III), glass-ionomer (Ketac Cem, liquid lot 638847, powder lot 642267, 3M ESPE, USA), and a 10-MDP-based cement (as in Study II). Five crowns per cement (n=5) in each group (As-sintered and KAPA) were cemented onto G10 dentin analogs using finger pressure and a following static load of 50N for 5 minutes. For the 10- MDP-based cement and initial light-curing for 2–3 seconds was done in order to facilitate the removal of excess cement. Next, each surface (mesial, distal, buccal, lingual, and occlusal) was light-cured for 20 seconds (TransLux Wave, Kulzer, Germany). The specimens were stored in deionized water for one year at 37 °C. The water was not changed during the whole year of ageing.

A universal testing machine (ZMART.PRO, Zwick/Roell, Germany) was used to evaluate the fracture strength. The specimens were placed in a water container with constant water circulation at 37 °C. A stainless steel piston (10 mm , spherical end) was advanced perpendicularly towards the occlusal surface at a rate of 1mm/min.

A 3.0-mm thick ethylene propylene diene sheet (Shore hardness 90)

was placed between the piston and the specimen to prevent

Hertzian/cone fracture on the occlusal surface.

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Fracture toughness and hardness (Study IV)

The three types of zirconia were used. Five plates (3mm x 3mm x 1mm) per type were prepared according to the ISO 14705:2016 (thickness > 0.5 mm). The plates were embedded in epoxy resin and polished as in Study III (the last polishing step was done using a cloth and a polishing suspension).

The Vickers indentations were made with the indenter DuraScan 70 G5 (Emco-test, Austria). The hardness was calculated by the software ecos Workflow Pro (Emco-test, Austria). The indentation force of 5 kgf produced consistent hardness values and well-defined cracks.

Two other indentation forces were also explored (1 kgf and 10 kgf) but were discarded due to inconsistent hardness values or complete fracture of the specimens. The type of fracture, median or Palmqvist, was determined by a) polishing and inspection (130) and b) measuring the proportion between the crack length (c or l) and the half diagonal length (a) (131,132). The cracks were found to be median. The elastic modulus was taken from the literature (47). Two equations were used:

Anstis et al.’s for median cracks (131) Kic = 0.016 • (E/HV) ^2/5 • (F/c ^3/2 )

where Kic = fracture toughness, E is the elastic modulus (GPa), HV the Vickers hardness (GPa), F the indentation force (N), and c the radial crack length (m).

Niihara et al.’s for median cracks (132) Kic = 0.129 • E ^2/5 • HV ^3/5 • Ø ^-3/5 • a ² • c ^-3/2

where Kic = fracture toughness, E is the elastic modulus (GPa), HV the Vickers hardness (GPa), Ø the constrain factor ( 3), a the half diagonal of the indentation (μm), and c is the radial crack length (μm).

The software ImageJ version 2.0-rc-43 (Open source image

processing software, Creative commons license) was used to measure

the indentations and the cracks from the images produced from the

ecos Workflow Pro software (Emco-test, Austria). In addition,

complementary scanning electron microscope (SEM) images were

also taken (LEO Ultra 55, Carl Zeiss, Germany). The specimens were

prepared for SEM as has been previously described.

3.1.2
Chemical Analyses

Raman and Fourier-transform infrared spectroscopies (Study II)

Presintered and isostatically pressed CAD/CAM blocks of the three zirconias were ground into fine powders. The powders were mixed with a 10-MDP-based cement (Panavia F 2.0 cement Lot: 000055, Kuraray, Japan) using a powder-cement weight ratio of 1:2 (108). The mixtures were formed into coins (3.0 mm 
x 1.0 mm) using a PMMA form (White Peaks, Germany), and the cement excess was removed using a polytetrafluoroethylene (PTFE or Teflon) spatula. A “coin”

containing only the 10-MDP-based cement served as a control. The light curing of the specimens was done for 20 seconds on each side (OptiLux VCL 500, Kerr, USA), following manufacturer’s recommendations. The Raman microscope (alpha300 R, WITec, Germany) used a Nd:YAG laser (532 nm wavelength), an optical objective of 100X, and an integration time of 10 seconds. The Fourier-transform infrared microscope (Hyperion3000, Bruker, Germany) used an attenuated total reflectance germanium objective in reflection mode. Both microscopes had an approximate depth of analysis of 5–10 μm. The data from each specimen (.txt or .xls) were paired in Microsoft Excel for Mac version 16.13 (Microsoft, USA).

The specimens were analyzed after 48h of storage in deionized water at 37 °C or after 10,000 thermocycles (5 °C–55 °C) (Huber, VWR, USA).

X-ray photoelectron spectroscopy (Study II)

The same specimen preparation was followed as described for FTIR and RS. Additionally, fully sintered zirconias were used as control materials, and the 10-MDP-based cement was also used as a control.

The analyses were conducted using the PHI 5000 VersaProbe III Scanning XPS Microprobe (Physical Electronics, USA) equipped with a monochromatic Al K
(h = 1486.6 eV) X-ray source. The ISO 15472 (133) was used to calibrate the registered binding energy.

Two peaks were used – the main peaks of gold (Au4f 7/2 , at 83.96 eV) and copper (Cu2p 3/2 , at 932.62 eV). Dual charge compensation was activated. Initial survey scans (0 to 1200 eV, step size 1.00 eV) were run. Next, narrow scans on the identified elements were conducted (step size 0.10 eV). The X-ray beam size was approximately 50 m.

Ion-etching was performed (argon, 81 Å/min) to remove surface

contamination. The spectra were compared before and after etching.

Sebastian Franco-Tabares

Back scattered electron microscopy and EDX (Study II) In Study II, in conjunction to SEM imagining, an energy selective backscattered detector (ESB) was used to get compositional information from the mixtures (LEO Ultra 55, Carl Zeiss, Germany).

The elements with higher atomic numbers appeared brighter and elements with lower atomic numbers appeared darker. Prior to ESB, the mixtures were polished using SiC papers (Struers, Denmark) under constant water (#500, #1000, #2000, and #2400).

Additionally, EDX analyses were performed on sites of interest – i.e., zirconia particle-agglomerations and filler particles of the 10-MDP- based cement.

X-ray diffraction (Study III)

From each group (KAPA 15 sec, KAPA 30 sec and polished), three plates (10 mm x 10 mm x 1.5/1.0 mm) of each zirconia type underwent X-ray diffraction (XRD). The diffractometer (SmartLab, Rigaku, Japan) used a Cu K radiation as the X-ray source at 45 kV of voltage with a current of 200 mA. The scan speed of 10°/min was complemented with a step size of 0.02°. The software TOPAS 5 (Coelho Software, Australia) (134) was used to implement the Rietveld analysis (135) to obtain the phase content of the specimens.

The models of each phase were acquired from the inorganic crystal structure database (ICSD) (https://icsd.fiz- karlsruhe.de/search/basic.xhtml) (10–13) (cubic (c), tetragonal (t), rhombohedral (r), and monoclinic (m), respectively). The atomic positions and the thermal parameters were fixed during the Rietveld refinement. The cubic phase required texture correction using spherical harmonics. The Rietveld refinement was accompanied by the estimation of the monoclinic volume fraction (Vm) using the Garvie and Nicholson method (136) modified by Toraya (137):

Vm = 1.311 x Xm/ (1+ 0.311 x Xm)

Xm = [Im(-111) + Im(111)]/[Im(-111) + Im(111) + It(101)]

where It is the integrated intensity of the tetragonal phase and Im is

the integrated intensity of the monoclinic phase. The integrated

intensities of both phases were estimated by the software PDXL

(Rigaku, Japan). The monoclinic phase volume fraction was

expressed as the percentage of the tetragonal phase.

3.1.3 Surface morphology analysis

Interferometry (Study III and Study IV)

Three specimens from each group were analyzed using a white-light interferometer (SmartWLI extended, Gbs, Germany) at three randomly selected points (Figure 12).

Figure 12. Illustration of the specimens analyzed using interferometry.

The interferometer used a 50X Mirau objective with a height resolution of 0.1 nm. An anti-vibration supporting device (Nanoseries, Accurion, Germany) situated under the interferometer was activated during the measurements. SmartVIS3D software version 2.1 (Gbs, Germany) was used to acquire surface data. The scanned area was 350220 μm. Subsequently, the data were processed by the software MountainMaps version 7.4 (Digital Surf, France). The processing of the data consisted of a high-pass Gaussian filter (50x50 m) as suggested by Wennerberg and Albrektsson (138).

The following variables were of interest: (a) Sa (μm) – i.e., average roughness; (b) Sdr (%) – i.e., additional surface area contributed by the roughness; and (c) Sds (1/μm ² ) – i.e., density of summits.

Additionally, in Study IV the variable Ssk (unitless) was studied as it

describes the valley or peak predominance of a surface. The surface

profile of Study III was analyzed using the software ImageJ version

2.0-rc-43 (Open source image processing software, Creative

commons license).